This invention relates to a transparent glass and, more particularly, to an aluminosilicate transparent glass having excellent corrosion resistance and chemical stability and being suitable for use as glasses for an inspection hole and glass for the level clairvoyance of a boiler.
Borosilicate glass or aluminosilicate glass is currently used for a level watching glass of a boiler or other apparatus. Borosilicate glass is generally used for uses in an environment under a low pressure and aluminosilicate glass is generally used for uses in an environment under a high pressure. Main requisites for glass for the level clairvoyance used in an environment under a high pressure are transparency in the visible light region, low thermal expansion property (xcex1: 50xc3x9710xe2x88x927/xc2x0 C. or below), high thermal shock, corrosion and water resistances and, particularly, high alkali resistance. Such glass must particularly have strictly high corrosion resistance under a caustic pressurized condition because sodium hydroxide or like alkali is mixed in an aqueous solution in a boiler whereby pH of the aqueous solution is in the order of 9.3 to 11.0.
The aluminosilicate glass used currently for uses in an environment under a high pressure have adequate low thermal expansion property and thermal shock resistance but its corrosion resistance under a caustic pressurized condition is not sufficient and, for this reason, this glass cannot be used directly as glass for the level clairvoyance. For coping with the problem of corrosion by alkali, mica or an alkali resistant glass is deposited on this glass for use as the glass for the level cliarvoyance (e.g., Japanese Patent Application Laid-open Publication No. 3-252331). The alkali resistant glass, however, contains Y2O3 which is a costly material and therefore raises the manufacturing cost of the glass. On the other hand, in the borosilicate glass which is used in an environment under a low pressure, the alkali resistant glass is not deposited on the glass for the reason that likelihood of corrosion is small in the environment in which the glass is used. In actuality, however, corrosion progresses significantly in an environment under pressure even if the pressure is of a relatively low level and such borosilicate glass is actually used with the pressure being reduced below a normal specification.
For these reasons, there is a demand for a transparent glass which has high chemical durability (i.e., water, acid and alkali resistances) and corrosion resistance and excellent low expansion and thermal shock resistance properties which can be used directly as the glass for the level clairvoyance without necessity for taking any extra step for coping with the problem of corrosion by alkali such as deposition of an alkali resistant glass.
It is, therefore, an object of the invention to provide a transparent aluminosilicate glass which is adapted for use in both an environment under a high pressure and an environment under a low pressure and, while maintaining the properties of the prior art aluminosilicate glass, namely high light transmittance in a visible light region, low thermal expansion property and high thermal shock resistance, additionally has such high chemical stability (i.e., water, acid and alkali resistances) and corrosion resistance that it is not necessary to deposit mica or alkali resistant glass on the glass and which can be melted at a relatively low temperature.
Accumulated studies and experiments made by the inventor of the present invention for achieving the above described object of the invention have resulted in the finding, which has led to the present invention, that a glass having high transmittance in the visible light region, low thermal expansion and thermal shock resistance properties and high chemical durability and corrosion resistance can be obtained in a SiO2xe2x80x94Al2O3xe2x80x94B2O3xe2x80x94CaOxe2x80x94MgOxe2x80x94BaOxe2x80x94SrOxe2x80x94ZnOxe2x80x94TiO2xe2x80x94ZrO2 glass.
According to the invention, there is provided a transparent glass comprising in mass percent on oxide basis:
In one aspect of the invention, the transparent glass has corrosion resistance in the form of mass loss of 4.0 mg or below per 1 cm2 of the surface of a specimen upon holding the specimen for five hours in sodium hydroxide solution having concentration of 300 mg/L under test pressure of 50 MPa at 250xc2x0 C.
In another aspect of the invention, the transparent glass has a coefficient of thermal expansion within a range from 35xc3x9710xe2x88x927/xc2x0 C. to 50xc3x9710xe2x88x927/xc2x0 C. within a temperature range from 100xc2x0 C. to 300xc2x0 C.
In another aspect of the invention, the transparent glass has powdered glass mass loss rate of 0.1% or below in terms of each of water resistance, acid resistance and alkali resistance.
In another aspect of the invention, the transparent glass has softening point within a range from 850xc2x0 C. to 950xc2x0 C.
According to the invention, there is provided a method for manufacturing the above described transparent glass wherein glass materials are melted at a temperature within a range from 1400xc2x0 C. to 1480xc2x0 C. and formed to glass.
There is also provided a method for manufacturing a transparent glass wherein the above described transparent glass is heated to a temperature below the softening point of the glass and thereafter is cooled and physically strengthened.